1. Field of the Invention
The present invention relates to telecommunications systems and, particularly, to an improved fault tolerant H.323 system.
2. Description of the Related Art
The International Telecommunications Union (ITU) Recommendation H.323 describes a set of devices and protocols for multimedia communication over packet-switched networks. The four main components defined by the specification are clients (also referred to as terminals or endpoints), multipoint control units, gateways and gatekeepers.
The H.323 Recommendation provides for separate H.225.0 call signaling, H.245 call control signaling and media channels. Typically, H.225.0 call signaling and H.245 call control signaling are routed through a gatekeeper, while the media channels (i.e., audio, data and/or video) are routed directly between the endpoints. For example, to place a call between two clients, the calling party client sends a message to the gatekeeper, which resolves the address of the receiving party and sends the appropriate signaling messages to the caller and receiver. Once the signaling and control channels have been established, the endpoints establish the media channels. The media channel passes directly between the endpoints. The H.225.0 signaling channel and H.245 call control channel are also used to terminate the call.
If the gatekeeper fails for any reason, the endpoints become aware of this because their connection to the gatekeeper (usually seen in the client as a TCP socket) will close. When the H.245 call control channel is detected as having been closed, the H.323 Recommendation requires the endpoints to disconnect the call and close the media channel(s). This requirement exists so that when the gatekeeper does recover, there are no outstanding media connections still alive of which the gatekeeper is unaware. Thus, the network is prevented from perpetually maintaining a call. When bandwidth management is employed, the network is prevented from allowing more calls than the network can handle.
While redundant back-up gatekeepers which track other gatekeepers' operations have been known to be provided, such systems do not allow for preservation of ongoing calls. In these cases, ongoing calls are lost and the H.323 client terminals must re-register and re-initiate call setup again, which can cause a burst of excessive load on the network and on the gatekeeper. Moreover, gatekeeper to gatekeeper communication can consume processing power and load the system.
The H.323 Recommendation provides for direct signaling between endpoints. If an endpoint knows the transport address of a destination endpoint, then the H.225.0 call signaling and H.245 call control channels can be established directly between the endpoints. However, direct signaling does not permit the full range of features users generally expect today from telephony systems, such as call transfer or forwarding, or bandwidth control.